Geology Reference
In-Depth Information
Fig. 9.13.
Model map, cross section and SCAT plots for a doubly plunging fold.
AP:
axial plane;
CP:
crestal
plane;
IP:
inflection plane. (After Bengtson 1981a)
L
dip with depth that gives the average plunge of zero. A uniformly plunging fold (Fig. 9.12)
plots as a line of constant plunge with depth. A doubly plunging fold (Fig. 9.13) shows the
plunge reversal with depth on the
L
component plot.
Dip-sequence analysis can be performed on a traverse in any direction through a
structure. As an example, the method is applied to a horizontal traverse across the map
of the Sequatchie anticline originally presented in Fig. 2.4. The traverse (Fig. 9.14) runs
from northwest to southeast at right angles to the fold axis along a stream valley that
provides the best exposure and therefore the most data. The traverse is broken into
three straight-line segments at the dashed lines in order to follow the valley. The atti-
tudes of bedding are located on the SCAT diagrams according to their distance from
the northwest end of the traverse. The numerical values are given in Table 9.1.
The
T
and
L
directions are determined from the tangent diagram and the dip-azimuth
diagram. The linear trend of dips on the tangent diagram (Fig. 9.15a) is the trend of
T
, and
L
is at right angles to it. On the dip-azimuth diagram (Fig. 9.15b) the two vertical lines of
points indicate, by comparison to Fig. 9.4, a non-plunging fold with a crest that trends 230.
The dip-azimuth diagram should always be checked against the tangent diagram before
finally deciding on the plunge direction and amount. Here the trend of the crest and the
lack of significant plunge agrees with the tangent diagram. The direction of the crest line,
here equal to the fold axis direction, is the
L
direction to be used in the next stage of the
analysis. The
T
direction is at 90° to
L
, parallel to the azimuth of the limb dip.
The SCAT diagrams reveal the details of the structure. The bedding azimuths and
dip components are plotted in Fig. 9.16. The azimuth and dip diagrams (Fig. 9.16a,b)
show the locations of the crestal plane, axial plane, and inflection plane (compare
